Efficiency switching voltage converter system

ABSTRACT

Disclosed is an improved efficiency switching voltage converter system wherein the semiconductor switching device employed therein is provided with increased gate drive by selectively applying the most effective driving voltage available in the system.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to switching voltage converter systems forconverting a D.C. input voltage to a different D.C. output voltage. Inparticular, this invention relates to switching voltage convertersystems employing semiconductor switches.

2. Background of the Invention

Switching voltage converter systems are employed in many applicationsand, in particular, are employed in D.C. applications to convert aninput voltage V_(IN) to a different D.C. output voltage V_(OUT) by meansof a switching action in the system. In one type of converter system,the voltage change results from the induced voltage in an inductor dueto the time variation of the current through the inductor due to theswitching action. The operation of such switching converters is wellknown in the art with the basic configuration of a prior art switchingconverter system for stepping up the voltage being shown in FIG. 1.

The switching means employed in modern switching voltage convertersystems is commonly a semiconductor switch such as a MOSFET, JFET orbipolar transistor or similar semiconductor device. Such semiconductorswitches typically employ appropriate switching logic for turning theswitch on and off as desired by applying the appropriate drive signal tothe semiconductor device. A step up switching voltage converter systememploying an n-channel MOSFET switch and suitable switching logic fordriving the switch gate is shown in FIG. 2. In various applications thesemiconductor switch and the switching control logic may be discretecircuits or form a part of a single monolithic IC semiconductor chip.

The efficiency of switching voltage converter systems is, in general,dependent upon the power losses in the circuit elements. In particular,power losses in the switch can have a significant effect on theefficiency of such converter systems. Where the switch of the systemoperates in a normal voltage range, the power loss in the switch will ingeneral be lower where the forward voltage drop across the switch isminimized. The specific reduction in power loss by reducing the voltagedrop across the switch and the corresponding increase in efficiencydepends on the specific type of semiconductor device employed as theswitch and the specific voltage range at which the system is operating.

The forward voltage drop across a semiconductor switch will typically bedependent upon the characteristics of both the switch device itself andthe drive signal applied to the switch. In many prior art switchingvoltage converter systems, the switching logic which supplies the switchdrive signal is powered by the input voltage V_(IN). For example, aMOSFET switch driven from the input voltage V_(IN) supplied to theswitching logic is shown in the prior art switching converter system ofFIG. 2.

For a given operating region of a MOSFET switch, such as the switchshown in FIG. 2, the voltage drop across the switch can be reduced byincreasing drive voltage between the gate and source (V_(GS)) of theswitch when it is on, the specific relationship depending on theparticular switch design and operating range, and other devicecharacteristics. This is also true for other FET switches and bipolartransistor switches (although in the latter case the voltage drop isreduced by increasing the current drive signal to the transistor base byincreasing the voltage). The device characteristics of the semiconductorswitch, although affecting the voltage drop across the switch, aregenerally not under the control of the circuit designer as this dependson the particular fabrication process used for the specificsemiconductor switch. However, one device characteristic which typicallyis under the designers' control is the size of the transistor switch,with the voltage drop across the switch in general decreasing withincreasing semiconductor switch size. More specifically, for a MOSFETswitch, the voltage drop across the switch will be inverselyproportional to the ratio of the device channel width to length. Makinga switch very large is however undesirable for economic and spaceconsiderations. °

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide animproved efficiency switching voltage converter system having reducedpower losses in the switch by increasing the drive signal to the switchduring operation of the system.

It is a further object of the present invention to provide a switchingvoltage converter system of a reduced size with no loss in systemefficiency.

The above and other objects are accomplished in a switching voltageconverter system in accordance with the present invention in which thedrive signal to the semiconductor switch of the system is a function ofeither the input or output voltage of the system to maximize the drivesignal. The particular means for implementing the present inventionvaries with the specific switching voltage converter topology to whichit is being applied. In a simple embodiment of the present invention, avoltage drive to the switch or switching logic is provided from both thevoltage input and voltage output. In an alternative embodiment, theinput voltage and output voltage are sampled by comparison means whichprovides the greater of these to the switch by turning on theappropriate one of two semiconductor devices. In an application wherethe output voltage is negative, i.e., in a switching voltage inverterapplication, the more negative of the output voltage and ground will besupplied to drive the switch.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of a simple prior art step up switchingvoltage converter.

FIG. 2 is a schematic diagram of a prior art step up converter systememploying an n-channel MOSFET as the converter switch.

FIG. 3 is a schematic diagram of a simple embodiment of the presentinvention in a step up switching voltage converter.

FIG. 4 is a schematic diagram showing an alternate embodiment of thepresent invention in a step up switching voltage converter.

FIG. 5 is a schematic diagram showing still another alternate embodimentof the present invention in an inverting switching voltage converter.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Switching voltage converter systems are employed in a wide variety ofapplications, and in particular DC applications, to convert an inputvoltage V_(IN) to a different output voltage V_(OUT). Many differentuses are possible for such systems, and V_(OUT) may be greater than(step up converter), less than (step down) or inverted (inverter) inrelation to V_(IN). Furthermore, for each type of converter manydifferent circuit topologies are possible.

The basic operation of switching voltage converter systems is well knownin the art and will only be briefly reviewed. FIG. 1 shows a simpleprior art step up switching voltage converter for converting an inputvoltage V_(IN) received at the voltage input 11 to an output voltageV_(OUT) provided to the voltage output 13. The system of FIG. 1 employsan inductor 10 through which current flows to system ground 19 whenswitch 12 is closed. Since the input is effectively coupled to systemground 19 through the inductor 10 when switch 12 is closed, the currenti_(L) through inductor 10 will be relatively high. Opening switch 12interrupts current flow through the low resistance path to system ground19 and current flows substantially only through diode 14 into capacitor16 and to voltage output 13. Immediately after switch 12 opens, thecurrent flow through inductor 10 will begin to decrease due to theincreased resistance of the circuit path through diode 14. This currentchange in inductor 10 results in an induced voltage in inductor 10proportional to Ldi_(L) /dt (where L is the inductance of inductor 10).The sign of the induced voltage is additive to V_(IN) resulting inV_(OUT) being greater than V_(IN) (ignoring the voltage drop acrossdiode 14). The function of capacitor 16 is to maintain V_(OUT) duringthe period switch 12 is closed and thereby smooth the value of outputvoltage V_(OUT).

FIG. 2 shows the step up switching voltage converter of FIG. 1 employinga semiconductor switch 18 in conjunction with appropriate switchinglogic 15 to accomplish the switching function. The specificsemiconductor switch 18 shown in FIG. 2 is an n-channel enhancementMOSFET, a form of semiconductor switch commonly used in switchingvoltage converter systems. Other types of semiconductor switches such asJFET, bipolar transistors or triacs may also be employed, however. Thespecific switching logic 15 employed may also take a variety of formswell known in the art. For example, commercially available IC switchinglogic such as Texas Instruments model TL 497 switching regulator circuitor Raytheon Semiconductor model RC 4193 NB switching regulator circuitare suitable. The specific form of switching logic does not affect thesubstance of the present invention.

As shown in FIG. 2, in the prior art switching voltage converter system,the drive signal to the semiconductor switch 18 is supplied fromswitching logic 15. More specifically, the drive voltage between thegate and source of the MOSFET switch 18 is provided by the switchinglogic 15. Switching logic 15 in turn is driven by V_(IN) supplied alongconductive line 17 to logic input 23 during start up of the system andthereafter. Switching logic 15 is also coupled to system ground 19 byline 21. The drive voltage supplied to MOSFET switch 18 by switchinglogic 15 is typically limited by the magnitude of V_(IN). For thepurposes of explanation, the details of the switching logic 15 may beignored and it may be treated as simply selectively providing the signalreceived along line 17 to line 26 to drive switch 18.

FIG. 3 shows a simple embodiment of the present invention employed in astep up switching voltage converter of the type shown in FIG. 2. Theembodiment shown in FIG. 3 adds a line 20, coupling the voltage supplyinput 23a of switching logic 15a to V_(OUT), and line 22 and diode 24,in place of the direct connection of V_(IN) to the switching logic 15a.In this embodiment, the switching logic 15a may be identical to or maydiffer from the switching logic 15 of FIG. 2, in accordance with therequirements of the particular application.

Basic circuit theory shows that lines 20 and 22 will thus provide to theswitching logic 15a the greater of V_(OUT) or V_(IN) (less the voltagedrop across diode 24) which will in turn drive switch 18. Diode 24 isprovided to prevent reverse current flow along line 22 when V_(OUT)exceeds V_(IN). Since the embodiment illustrated in FIG. 3 is in a stepup converter application, V_(IN) will generally exceed V_(OUT) onlyduring start up. Thereafter, V_(OUT) will exceed V_(IN) (possibly by alarge factor) and will drive switch 18, through switching logic 15a.

Providing the greater of V_(IN) and V_(OUT) to the switching logic 15aand thereby to drive the semiconductor switch 18, allows significantgains in circuit efficiency with an easily implemented modification ofthe switching voltage converter circuit. In particular, where thesemiconductor switch 18 is discrete from the switching logic 15a, theembodiment of FIG. 3 may provide significant increases in the efficiencyof the switching voltage converter system with a straightforward designmodification. This increase in efficiency results from the devicecharacteristic of FET switches that, in the normal operational region ofthe transistor, increasing the drive voltage between the gate and sourcewill reduce the voltage drop across the switch. This reduction involtage drop across the switch will, for operation in the normal voltagerange for the switch, reduce the power loss in the switch. The specificpercentage change in efficiency which results from a configuration suchas in FIG. 3 will depend on the specific values of V_(IN) and V_(OUT)and the specific device characteristics of semiconductor switch 18.Preliminary indications, however, indicate that improvements inefficiency as high as 50% may be achieved for certain applications ofthe present invention as shown in FIG. 3.

FIG. 4 shows an alternate embodiment of the present invention still inthe context of a step up switching voltage converter system. Theembodiment shown in FIG. 4 employs a comparator 28 in conjunction withtwo (2) semiconductor switch devices 30, 32 to provide respectively thegreater of V_(IN) and V_(OUT) to semiconductor switch 18. The embodimentshown in FIG. 4 although more complex than that shown in FIG. 3 will ingeneral be capable of further increasing the efficiency of the step upswitching voltage converter system during start up. This additionalefficiency increase is due to the capability of substantially reducingthe forward voltage drop across first semiconductor device 30 comparedto that of the diode 24 shown in FIG. 3. Therefore, during start up ofthe switching voltage converter, the voltage supplied to the gate ofsemiconductor switch 18 will be greater by the corresponding differencein voltage drops.

A second advantage of the alternate embodiment shown in FIG. 4 is thepossibility of fabricating the entire switching voltage converter, withthe exception of the inductor 10, diode 14 and capacitor 16 on a singleCMOS integrated circuit chip. Therefore, while somewhat more complexthan the embodiment in FIG. 3, in applications where a monolithic ICdesign is suitable, the embodiment in FIG. 4 can in general be moreadvantageous. In such applications, however, special consideration mustbe given to maintaining the p-channel substrates at the correctpotential.

Referring to FIG. 4, the specific embodiment is shown employing a MOSFETsemiconductor switch 18 and MOSFETs 30, 32. JFET or bipolarsemiconductor switches can be used, however, with the appropriate designchanges. During start up of the switching voltage converter system,V_(IN) will be the most positive potential in the circuit and would bethe desired driving potential for semiconductor switch 18. At this time,parasitic transistor 34, shown explicitly in FIG. 4 across MOSFET 30 ina diode connected form, will be forward biased by V_(IN). (Transistor 34is characterised as parasitic due to its natural presence in thefabrication of a MOSFET. Although in normal applications the parasitictransistor is inactive and not explicitly shown, in this particularapplication it is active during start up of the system and therefore isshown explicitly across MOSFET 30). Parasitic transistor 34 activated inthe diode connected form thus passes current along line 36 at thepotential V_(IN) less the diode voltage drop across parasitic transistor34. This voltage is supplied along line 36 to the switching logic 15band to the gate of switch 18 and along line 40 to provide power tocomparator 28. At this point, comparator 28 is activated and will switchfirst MOSFET 30 on and, via inverter 42, MOSFET 32 off. As MOSFET 30switches on, current is supplied to switching logic 15b along line 36 toinput 23b and thereby, via switching logic 15b, to switch 18 at thepotential V_(IN) less the relatively small voltage drop across MOSFET30. In a monolithic CMOS IC application, this potential will be suppliedto the appropriate p-channel substrates of the IC chip containing theMOSFET 30, comparator 28, MOSFET 32, inverter 42, switching logic 15band switch 18.

As the switching voltage converter system begins operating V_(OUT) willrise above V_(IN) at which point comparator 28 will turn off MOSFET 30and turn on MOSFET 32. When MOSFET 32 has been turned on the switchinglogic 15b and switch 18 will thus be supplied with V_(OUT) less therelatively small voltage drop across MOSFET 32. In a monolithic CMOS ICimplementation embodiment this voltage will be supplied to theappropriate p-channel substrates thereby providing maximum gate tosource voltage to all MOSFET circuit elements, and in particular toMOSFET switch 18.

FIG. 5 shows another embodiment of the present invention in anapplication of a switching voltage converter operating as an inverter.In other words, for V_(IN) positive, V_(OUT) will be negative. The basicoperation of a switching voltage inverter, of which FIG. 5 represents animprovement over, is well known in the art and operates on the sameprinciple as the step up voltage converter; namely that the inductorforming part of its circuit will tend to maintain current flow constantduring switching changes and an induced voltage proportional to Ldi/dtwill be present in the circuit. Thus, in FIG. 5 when semiconductorswitch 44 is closed, a relatively large current will flow throughinductor 46 due to the low resistance path to ground 47. Diode 50 isreverse biased due to the positive voltage applied by V_(IN). Whenswitch 44 is opened the decreasing current through inductor 46 resultsin an induced voltage proportion to Ldi/dt, diode 50 becomes forwardbiased and negative V_(OUT) is provided. This current flow will alsocharge the bottom plate 60 of capacitor 48 positively and the upperplate 62 negatively. Capacitor 48 provides a smoothing function bymaintaining V_(OUT) during the period switch 44 is closed.

The operation of the present invention as disclosed in the embodiment ofFIG. 5 in a switching voltage inverter application is essentially thesame as in the step up converter application, i.e., voltages availablein the circuit are tested to determine the voltage to be used to drivethe semiconductor switch 44. In a voltage inverter application, however,V_(OUT) is compared to ground to determine which is more negative andthe more negative of V_(OUT) and ground is then supplied to theswitching logic 49 which supplies the drive signal along line 51 to thegate of p-channel MOSFET switch 44. Switch 44, being a p-channel MOSFETin this specific embodiment, will in general have a decreased voltagedrop and less power loss for increased gate to source voltagedifference. Therefore, as in the case of the step up voltage converterthe efficiency of the circuit will be increased by increasing the gateto source drive signal; the specific increase depending on the specificvoltage range and device charateristics.

Referring to FIG. 5, the operation of the embodiment shown is analogousto that of the step up converter application shown in FIG. 4. Comparator52 compares V_(OUT) and ground to determine which is more negative. Twon-channel MOSFETs 54 and 56 are employed in a manner analogous to the 2p-channel MOSFETS shown in FIG. 4. A parasitic NPN transistor 58 isexplicitly shown across MOSFET 54 in diode connected form. As in thecase of the parasitic transistor in the p-channel MOSFET of FIG. 4, theparasitic transistor 58 is inherently present due to the fabrication ofthe n-channel MOSFET 54, although in a normal application it is notactive.

During initial start-up of the switching voltage inverter of FIG. 5, theparasitic NPN transistor 58 operating as a diode will provide the groundpotential plus the diode voltage drop of transistor 58 to the switchinglogic 49 and n-channel MOSFET switch 44. As the circuit commencesoperating comparator 52 will be turned on. Initially ground will be morenegative than V_(OUT) and comparator 52 will enable first MOSFET 54thereby providing the ground potential plus the device voltage drop tothe switching logic 49 and switch 44. In the monolithic IC application,the ground potential will be provided to the appropriate n-channelsubstrates. As V_(OUT) becomes more negative than ground, comparator 52will switch off first n-channel MOSFET 54 and turn on second n-channelMOSFET 56 via the inverter 60. Second MOSFET 56 provides V_(OUT) plusthe voltage drop across second MOSFET 56 to switching logic 49 andswitching logic 49 in turn selectively applies this along line 51 todrive switch 44. The gate to source drive signal potential in switch 44will therefore be the potential difference between V_(IN) (positive) andthe more negative of V_(OUT) and ground (ignoring any losses in theswitching logic 49). This maximizes the drive to switch 44 therebyreducing power loss in switch 44 and increasing the circuit efficiency.In the monolithic CMOS IC application the n-channel substrates will beat the more negative of V_(OUT) and ground, assuring maximum gate tosource drive signal to all the MOSFET elements and the optimumefficiency of the switching voltage inverter system.

While the present invention has been described in terms of preferredembodiments in step up and inverting switching voltage converterapplications, it will be appreciated that the present invention isequally applicable to other switching voltage converter uses and circuittopologies. Also, while the preferred embodiments described above havebeen in the context of switching voltage converter systems employing asingle semiconductor switch, the present invention is equally suitableto systems employing more than one switch such as transformer-coupledpush-pull DC-to-DC converter systems. Furthermore, while theabove-described preferred embodiments employ MOSFET switches, thepresent invention is equally applicable to switching voltage convertersemploying bipolar or JFET switches. The necessary design changes in thecircuitry will be apparent to one skilled in the art. Similarly,although the MOSFETs illustrated in the preferred embodiments have beenenhancement type, depletion type MOSFETs could also be employed, withappropriate changes in the circuit design.

It will be apparent to one skilled in the art that other changes in thedetails of the preferred embodiments described above may be made andsuch alternate embodiments are within the scope of the presentinvention. Thus, the present invention is not intended to be limited tothe above described preferred embodiment and is instead best describedby the following claims.

What is claimed is:
 1. A switching voltage converter systemcomprising:voltage input means for receiving an input voltage;semiconductor switch means coupled to said voltage input means forselectively allowing current flow through said switch means; switchdrive logic means coupled to said switch means for supplying a drivesignal to said semiconductor switch means and causing the selectiveoperation thereof; voltage output means coupled to said semiconductorswitch means for providing an output voltage; and selective voltagesupply means coupled to said voltage input means and said voltage outputmeans for supplying the greater of said input voltage and said outputvoltage to said switch drive logic means wherein the drive signal tosaid switch means may be maximized to reduce energy losses through saidswitch means.
 2. A switching voltage converter system as set out inclaim 1 wherein said selective voltage supply means furthercomprises:comparator means for comparing said input voltage and saidoutput voltage and supplying a first signal when said input voltage isgreater than said output voltage and a second signal when said inputvoltage is less than said output voltage; first semiconductor meansresponsive to said comparator means for supplying said input voltage tosaid switch drive logic means upon receipt of said first signal; andsecond semiconductor means responsive to said comparator means forsupplying said output voltage to said switch drive logic means uponreceipt of said second signal.
 3. A switching voltage system as set outin claim 2 wherein said first semiconductor means comprises a first MOStransistor and said second semiconductor means comprises a second MOStransistor and an inverter connected in series with the gate of saidsecond MOS transistor.
 4. A switching voltage converter system as setout in claim 1 wherein said selective voltage supply means comprises adiode coupling said voltage input means and said switch drive logicmeans and a conductive line coupling said voltage output means and saidswitch drive logic means.
 5. A switching voltage converter systemcomprising:voltage input means for receiving an input voltage; switchingmeans having an input terminal, said switching means coupled to saidvoltage input means for selectively allowing current flow therethroughin response to a signal supplied to said input terminal; voltage outputmeans coupled to said switching means for providing an output voltage;and means coupled to said voltage input means and said voltage outputmeans for selectively providing the greater of said input voltage andsaid output voltage to said input terminal of said switching means todrive said switching means.
 6. A switching voltage converter system asset out in claim 5 wherein said means for selectively providingcomprises first voltage supply means for providing voltage from saidvoltage input means when said input voltage exceeds said output voltageand second voltage supply means for providing voltage from said voltageoutput means when said output voltage exceeds said input voltage.
 7. Aswitching voltage converter system as set out in claim 5 wherein saidmeans for selectively providing comprises:comparator means for comparingsaid input voltage and said output voltage and supplying a first signalwhen said input voltage is greater in magnitude than said output voltageand a second signal when said output voltage is greater in magnitudethan said input voltage; first semiconductor means responsive to saidcomparator means for providing said input voltage to said switchingmeans during start up of the converter system and thereafter in responseto said first signal; and second semiconductor means responsive to saidcomparator means for providing said output voltage to said switchingmeans in response to said second signal.
 8. A switching voltageconverter system as set out in claim 7 wherein said second semiconductormeans includes a first MOS transistor and an inverter connected inseries with the gate thereof and wherein said first semiconductor meanscomprises a second MOS transistor having a parasitic bipolar transistorconnected in diode form across the source and drain of said second MOStransistor.
 9. A switching voltage converter system comprising:a voltageinput for receiving an input voltage; an inductor coupled to saidvoltage input; a semiconductor switch coupled to said inductor andsystem ground for selectively allowing current flow through saidinductor between said voltage input and the system ground; logic meanscoupled to said switch for selectively opening and closing said switchby providing a drive signal thereto; a voltage output coupled to saidinductor and capacitively coupled to said switch and system ground foroutputting an output voltage; a comparator coupled to said voltage inputand said voltage output for comparing the magnitude of said inputvoltage and said output voltage and providing a first signal when saidinput voltage exceeds said output voltage and a second signal when saidoutput voltage exceeds said input voltage; a first semiconductor devicecoupled to said voltage input and said logic means and responsive tosaid comparator for supplying voltage to said logic means from saidvoltage input upon start up of the converter system and thereafter uponreceipt of said first signal; and a second semiconductor device coupledto said voltage output and said logic means and responsive to saidcomparator for supplying voltage to said logic means from said voltageoutput upon receipt of said second signal.
 10. In a dc voltage convertercircuit of the type to which an input voltage V_(in) is applied to aninput terminal and in which there is an output voltage V_(out) providedat an output terminal, and which has, operatively connected between theinput terminal and the output terminal an inductor, a rectifier and avoltage controlled semiconductor switch having a gate terminal andsource and drain terminals connected in series with said rectifier,wherein said switch is operated at a certain duty cycle and saidinductor, rectifier and switch cooperate to supply at the outputterminal a dc voltage, the improvement comprising:means for selectivelydelivering to the gate terminal of said semiconductor switch the greaterone of said output voltage V_(out) and said input voltage V_(in) asmeasured respectively with respect to the source terminal of saidsemiconductor switch as a reference.